Sorry, I didnt understand properly before. Ok well.. This variable will post a particular field or image so to speak like $post[field10] etc.
The variable is also used in templates such as "postbit_legacy"
Do a search to see the uses, but here are some examples:
$post[musername]
$post[usertitle]
$post[avatarurl]
$post['joindate']
$post[signature]
Hope this helps...
Out of interest, do you have xfire? I would be happy to help you on this one if needed
Regards
Matt